1. Field of the Invention
This invention relates in general to arrays of miniature ultrasonic transducers, and in particular to ultrasonic imagers having an array of sensors on a micromachined support substrate.
2. Discussion
The need for accurate ultrasonic sensors has grown with advances in medical diagnosis and other diagnostic fields. With well-developed silicon integrated circuit technology available for design and fabrication purposes, a large number of small size transducers may be fabricated on a wafer substrate, with the potential for further integration of on-chip signal processing circuitry. Large arrays of small size transducers help improve image quality, which is important in medical imaging and non-destructive evaluation. See, D. W. Fitting, IEEE Trans. Ultrason. Ferroelec. Freq. Contr., UFFC-34, p.346 (1987).
The use of the piezoelectric polymer polyvinylidene difluoride (PVDF) material has been of interest in acoustic imaging and non-destructive evaluation since its discovery because of its strong piezoelectricity, low acoustic impedance (small mismatch with those of water and biological tissues) and flexibility. See H. Kawai, "The Piezoelectricity of Polyvinylidene Fluoride," Japan Journal of Applied Physics, Vol. 8, p. 975 (1969).
There has been great interest in the use of PVDF transducers mounted on substrates with conventional integrated circuit technology. See R. G. Schwartz & J. D. Plummer, "Integrated Silicon PVDF Acoustic Transducer Arrays," IEEE Trans. on Electron Devices, Vol. ED-26, pp. 1921-1931 (1979). Applications of PVDF on ultrasonic sensors have provided improved bandwidth and greater sensitivity and acceptance angle in a liquid environment.
Recently, another piezoelectric material, a copolymer of vinylidene fluoride and trifluorethylene known as P(VDF-TrFE), has received attention for use in solid-state ultrasonic sensing arrays, since it is more compatible with conventional integrated circuit processing technology. See, T. Furukawa et al, "Ferroelectric behavior in the copolymer of vinylidene fluoride and trifluoroethylene," Japan J. Appl. Phys., Vol. 19, pp. L109-L112 (1980); T. Yamada et al, "Ferroelectric to paraelectric phase transition of vinylidene fluoride-trifluoroethylene copolymer," J. Appl. Phys., Vol. 52(2), pp. 948-952 (1981); H. Ohigashi et al, Piezoelectric and ferroelectric properties of P(VDF-TrFE) copolymers and their application to ultrasonic transducers," Ferroelectrics, Vol. 69, pp. 263-276 (1984). Its piezoelectricity and acoustic impedance appears to be comparable to or even somewhat superior to those of PVDF for ultrasonic transducers. See, A. Fiorillo et al, "Spinned P(VDF-TrFE) copolymer layer for a silicon-piezoelectric integrated US transducer," 1987 Ultrasonics Symposium, pp.667-670 (1987). This copolymer can be spun on a silicon wafer, poled, and patterned and etched with a reactive ion etch (RIE). See, e.g., N. Yamauchi, "A metal-insulator-semiconductor (MIS) device using a ferroelectric polymer thin film in the gate insulator," J. Appl. Phys., Vol. 25(4), pp. 590-594 (1986).
A well-known technique in the ultrasonic sensing array arts for helping reduce electrical and acoustical cross-coupling effects between neighboring elements of the sensing array involves isolating the active transducer elements from one another by etching away the piezoelectric material in between the elements. See, C. Bruneel et al, "Electrical coupling effects in an ultrasonic transducer array," Ultrasonics, (November, 1989).
However, current large array ultrasonic sensors, mounted on silicon substrates and utilizing PVDF or P(VDF-TrFE), still have several shortcomings which limit desirable performance. A large parasitic capacitance between the lower electrode of the sensor and conductive (or semi-conductive) substrate on which it is mounted shunts the input to the processing circuitry and causes sensitivity loss. Also, lateral propagation of electrical signals and acoustic waves causes crosstalk between elements in the sensor array. This is illustrated in FIG. 1, which is a simplified cross-sectional diagram of a silicon semiconductor substrate 1 with several ultrasonic sensing elements 2 on its top surface. The substrate 1 is thick enough (e.g., around 150 to 500 microns) to sustain bulk waves at typical diagnostic ultrasonic frequencies (e.g., 1 MHz through 50 MHz). FIG. 1 shows that a single incoming wave 3 can generate a large number of reverberations 4 and 5, and remote wave leakage, represented by arrows 6, 7 and 8. This occurs because single-crystal silicon is a relatively unattenuating material. Note that the FIG. 1 diagram only shows longitudinal waves, and neglects shear and surface waves, which further compound this problem of crosstalk. Finally, the high propagation velocity of acoustic waves in silicon substrate may seriously limit the acceptance angle of a transducer array through crosstalk. As the size of each sensor elements is diminished for greater integration, any sensitivity loss from already small signals degrades performance.
One possible way of overcoming some of the foregoing problems is to increase radiated ultrasonic power, so that the reflected signals from the object to be detected are stronger, and therefore may be more easily distinguished from one another. However, in many biomedical applications, ultrasound procedures requiring fine resolution of soft internal tissue structures such as organs within the human body are already being carried out at the maximum allowed power. Thus, simply increasing the ultrasonic power input into such internal tissue structures to further improve image resolution structures is not possible. In order to effect higher resolution images, some other improvements in the signal-to-noise ratios produced by ultrasonic image sensing arrays are therefore required. In other words, sensing arrays must be designed and constructed to produce a higher resolution image for a given input power level if ultrasonic biomedical imaging of soft tissue structures is to improve.
In light of the foregoing problems and shortcomings, it is an object of the present invention to provide a high performance multi-element ultrasonic sensor array for use in applications where high density and accuracy are important.
A further object of the present invention is to provide a multi-element ultrasonic transducer array which provides better image quality by greatly reducing the parasitic capacitance between sensor electrodes and substrates, and yielding an increased signal output.
Yet another object of the present invention is to reduce crosstalk between neighboring sensor elements, which also increases image accuracy and acceptance angle of the array. A related object is to improve the signal-to-noise ratio of ultrasonic arrays, which also will permit higher resolution images to be obtained.
Still another object of the present invention is to increase the frequency range of signals which the ultrasonic sensor array may detect.
One more object is to provide a method to fabricate an ultrasonic sensor with a robust diaphragm and supporting structure that can tolerate the removal of the substrate under the sensor.